Event-based interaction is suitable for rescue and emergency applications because the filtering capabilities can help to prevent information overload, and such interaction may be offered by an Event Notification Service (ENS). We focus on ENS in sparse Mobile Ad-hoc Networks (MANETs), since an incident may occur e.g., in a deserted place lacking infrastructure, the density of nodes may be low, and there may be physical obstacles limiting the transmission range. The asynchronous communication provided by the ENS is suited for an environment where there may be long-lasting network partitions. In this chapter, we describe characteristics of rescue operations and use this as a basis for discussing ENS design choices such as subscription language, architecture and routing. Afterwards, we present our own ENS solution, the Distributed Event Notification Service (DENS), which is tailored for such an application domain.
TopIntroduction
Rescue operations may occur in environments where there is no communication infrastructure or where the infrastructure has been damaged by the incident itself. In addition to, or as a replacement to radio and telephones, handheld PDA-like devices can be used at the rescue site for communication. Such devices run applications and can spontaneously form a mobile ad-hoc network (MANET) using their wireless network interfaces. The purpose of setting up such a network is to exchange information. However, it is important to prevent information overload; the rescue personnel should be able to focus on the task of saving lives and not browse through a lot of information or search for information. They should be interrupted or notified only when something occurs that is of interest to them, and this type of interaction is suited to event-based interaction. Event-based communication may be offered by an event notification service (ENS) using the publish/subscribe communication paradigm. At the application level, instead of using classical client-server interaction, the application can describe events of interest in a subscription and is notified when such an event takes place by the ENS via a notification, i.e., the communication is asynchronous. This asynchronous communication service can be used by application developers to enable applications to adopt or react to certain events, or to directly notify the end-user about the event.
The main entities in an ENS are subscribers and publishers. Subscribers subscribe for information while publishers publish information concerning an event. They are decoupled by the ENS and can therefore communicate asynchronously. It is the task of the ENS to match subscriptions and notifications describing published events and deliver them to interested subscribers. The service itself may be centralized or distributed. ENSs differ in how events are observed, how subscriptions are expressed, i.e. the subscription language, how notifications are disseminated, and their overall architecture. The design is governed by the application domain and its requirements and the kind of network they are tailored for.
Important characteristics of rescue operations in this context are the heterogeneity inherent in the diversity of devices used, organizations, communication lines (information flows), applications, and the rescue operations themselves. These characteristics should be considered when deciding the requirements for the ENS for rescue and emergency in sparse MANETs. Some questions that arise in this context are:
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Reliability and Availability: To what extent must the service be available and what delivery semantics is needed?
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Persistency: For how long should notifications about events be stored?
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Resource awareness: What are the implications of the limited resources on handheld devices and in MANETs?
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Expressiveness: How to describe events and where to filter events?
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Adaptability: In what way may rescue operations differ and what impact may these differences have on the usage of the service and the network characteristics?
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Security: Does the presence of sensitive data such as medical records result in extra need for security compared to other application domains?
The rest of this chapter is organized as follows: We first describe the application scenario and network environment. We proceed by discussing design choices and related work. Next, we go a step beyond the state-of-the-art and present our approach, DENS, for use in rescue operations. We describe how its design and features take into account the design considerations. Finally, we make some concluding remarks and present future work.
TopBackground: Application Scenario And Network Environment
In the introduction, we motivated the usage of event-based interaction in mobile environments for rescue and emergency applications. In the following, we look more closely at the environment describing both the network itself and the characteristics of rescue operations. Then we revisit the questions mentioned in the introduction.